Carbide stabilized catalyst structures and method of making

ABSTRACT

A catalyst structure for an electrochemical cell includes a catalyst support structure, catalyst particles and an outer carbide film The catalyst particles are deposited on the catalyst support structure. The outer carbide film is formed on the catalyst support structure. The outer carbide film surrounds the catalyst particles.

BACKGROUND

Electrodes containing supported metal catalyst particles are used inelectrochemical cells, such as fuel cells. For example, in aconventional hydrogen fuel cell, a supported platinum catalyst is usedto oxidize hydrogen gas into protons and electrons at the anode of thefuel cell. At the cathode of the fuel cell, another supported platinumcatalyst triggers an oxygen reduction reaction (ORR), leading to theformation of water.

The catalyst support is typically a conductive high surface area carbon.The catalyst support provides a surface over which the catalystparticles are dispersed and stabilized. However, the carbon support mayhave poor interactions with the catalyst particles, which results inchanges in the properties of the electrode. More specifically, poorcatalyst-support interactions results in particle size growth of thecatalyst particles under dissolution/redeposition processes. Theincrease in size of the catalyst particles throughdissolution/redeposition causes a loss in fuel cell performance. Thepoor interactions between the carbon and the catalyst particles can evenresult in irreversible loss of the catalyst in the cathode.

Additionally, carbon catalyst supports in fuel cells are susceptible tocorrosion that results in carbon oxidation and, as a final stage,collapse of the carbon pore structure. Causes of corrosion include thepresence of oxygen, water, and high electrode potential, especially onthe cathode side. Corrosion also causes microstructural derogation andsurface chemistry changes, which can result in an irreversible loss incatalyst performance, cross-over and ultimately in the complete failureof the fuel cell. An improved catalyst support is needed so that theperformance of an electrochemical cell can be maintained.

SUMMARY

A catalyst structure for an electrochemical cell includes a catalystsupport structure, catalyst particles and an outer carbide film. Thecatalyst particles are deposited on the catalyst support structure. Theouter carbide film is formed on the catalyst support structure. Theouter carbide film surrounds the catalyst particles.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic of a fuel cell that uses the catalyst structuresdescribed herein.

FIG. 2 is a cross-sectional view of a catalyst structure having acarbide thin film for use in the fuel cell of FIG. 1.

FIG. 3 is a block diagram illustrating a method for producing thestabilized catalyst structure of FIG. 2.

FIG. 4A is a block diagram illustrating a method for depositing acarbide thin film by microwave irradiation.

FIG. 4B is a block diagram illustrating an alternative method fordepositing a carbide thin film by microwave irradiation.

FIG. 5 is a cross-sectional view of an alternative catalyst structurehaving a carbide thin film.

It is noted that the figures are not to scale.

DETAILED DESCRIPTION

A stabilized catalyst structure is described herein which includescatalyst particles stabilized on a carbon support by a carbide thinfilm. The stabilized catalyst structures can be used in fuel cells andother electrochemical devices. For example, the stabilized catalyststructures can form the basis for electrochemical cell catalyst layers.

FIG. 1 is an example fuel cell 10, designed for generating electricalenergy, that includes anode gas diffusion layer (GDL) 12, anode catalystlayer 14, electrolyte 16, cathode gas diffusion layer (GDL) 18, andcathode catalyst layer 20. Anode GDL 12 faces anode flow field 22 andcathode GDL 18 faces cathode flow field 24. In one example, fuel cell 10is a fuel cell using hydrogen as fuel and oxygen as oxidant. It isrecognized that other types of fuels and oxidants may be used in fuelcell 10.

Anode GDL 12 receives hydrogen gas (H₂) by way of anode flow field 22.Catalyst layer 14, which may be a platinum catalyst, causes the hydrogenmolecules to split into protons (H⁺) and electrons (e⁻). Whileelectrolyte 16 allows the protons to pass through to cathode 18, theelectrons travel through an external circuit 26, resulting in aproduction of electrical power. Air or pure oxygen (O₂) is supplied tocathode 18 through cathode flow field 24. At cathode catalyst layer 20,oxygen molecules react with the protons from anode catalyst layer 14 toform water (H₂O), which then exits fuel cell 10, along with excess heat.

Catalyst particles dispersed and stabilized on catalyst supportstructures can form the basis of anode catalyst layer 14 and cathodecatalyst layer 20. In one example, the catalyst particles are platinum.As described above, cathode catalyst layer 20 is used to increase therate of the oxygen reduction reaction (ORR) ultimately resulting in theformation of water from protons, electrons and oxygen. Cathode catalystlayer 20 contains platinum as a catalyst but the platinum is unstable inthis environment. During potential cycling, platinum atoms tend todissolve and redeposit. This dissolution/redeposition process results incatalyst particle growth that decreases the performance of the fuelcell.

In one example, fuel cell 10 is a polymer electrolyte membrane (PEM)fuel cell, in which case electrolyte 16 is a proton exchange membraneformed from a solid polymer. In another example, fuel cell 10 is aphosphoric acid fuel cell, and electrolyte 16 is liquid phosphoric acid,which is typically held within a ceramic (electrically insulating)matrix.

FIG. 2 is a cross-sectional view of stabilized catalyst structure 30 foruse, for example, as a basis for anode catalyst layer 14 and cathodecatalyst layer 20 in fuel cell 10. Catalyst structure 30 includes carbonsupport or core 32, catalyst particles 34 and outer carbide thin film36. Carbon support 32 is any suitable carbon containing support. In oneexample, carbon support 32 is activated carbon or carbon black, such asKetjen Black (KB). In another example, carbon support 32 includesmesoporous carbons, surface modified carbons (or modified carbons),carbon nanotubes, carbon nanowires, graphitized carbon, non-graphitizedcarbon and combinations thereof. Carbon support 32 is a catalyst supportstructure that supports catalyst particles 34.

Catalyst particles 34 can include any catalyst, such as platinum, gold,iridium, osmium, palladium, rhodium and ruthenium and mixtures andalloys thereof Catalyst particles 34 can be a mixture of pure elementsand/or alloys. Depositing a mixture of different elements and/or alloyscan enhance the catalyst activity and stability of catalyst particles34.

Catalyst particles 34 are nanoparticles. In one example, catalystparticles 34 have a diameter between about 0.2 nanometers (nm) and about100 nm. In another example, catalyst particles 34 have a diameterbetween about 1 nm and about 100 nm. In a further example, catalystparticles 34 have a diameter between about 0.2 nm and about 20 nm. In astill further example, catalyst particles 34 have a diameter betweenabout 0.2 nm and about 10 nm. In a still further example, catalystparticles 34 have a diameter between about 2 nm and about 10 nm.

Catalyst particles 34 are deposited on carbon support 32 using knowndeposition techniques to form supported catalyst particles. In oneexample, a chemical deposition process is used. In a specific example,the chemical deposition process includes dissolution of a platinumprecursor and deposition of platinum particles by a reducing agent suchas H₂COH, H₄N₂ or NaBH₄. Other catalyst particle synthesis anddeposition techniques can be used, such as electrochemical depositionand thermo-decomposition techniques.

After catalyst particles 34 are deposited on the catalyst supportstructure of carbon support 32, outer carbide thin film 36 is formed oncarbon support 32. Outer carbide thin film 36 preferentially forms oncarbon support 32. Outer carbide thin film 36 primarily surroundscatalyst particles 34 and anchors catalyst particles 34 to carbonsupport 32. However, a small amount of outer carbide thin film 36 mayform on catalyst particles 34. Formation of outer carbide thin film 36on catalyst particles 34 is not preferable because it reduces thecatalytic activity of catalyst particles 34, as described further below.

Outer carbide thin film 36 can include tungsten carbide, boron carbide,niobium carbide, molybdenum carbide, tantalum carbide, titanium carbideand combinations thereof. Outer carbide thin film 36 is an atomicallythin layer. In one example, outer carbide thin film 36 has a thicknessof between about 1 and 2 atomic layers.

In another example, outer carbide thin film 36 has a thickness of lessthan about 1 nanometer. Outer carbide thin film 36 can be a homogeneousor heterogeneous layer. For example, outer carbide thin film 36 can havea uniform or non-uniform thickness. In one example, outer carbide thinfilm 36 can be thicker in selected locations compared to otherlocations. In another example, outer carbide thin film 36 can be smallclusters so that a portion of the catalyst support structure may not becovered by outer carbide thin film 36.

Outer carbide thin film 36 stabilizes catalyst particles 34 on carbonsupport 32. As the diameter of catalyst particles 34 becomes smaller,particles 34 become more unstable on carbon support 32. For example,platinum nanoparticles having a diameter of about 2 nanometers or lessare very unstable on carbon black. Such platinum nanoparticles are inmotion on the carbon black due to the high surface energy, and quicklyform agglomerates, which reduce the catalytic activity of the supportedcatalyst particles. Outer carbide thin film 36 physically anchorscatalyst particles 34 on carbon support 32, and prevents or reducesmovement of catalyst particles 34 on carbon support 32. Applying outercarbide thin film 36 after catalyst particles 34 are deposited on carbonsupport 32 stabilizes catalyst particles 34 and may allow catalystparticles 34 having diameters of about 2 nm or less to be used in fuelcell applications.

Outer carbide thin film 36 also protects carbon support 32 fromcorrosion. In a fuel cell environment, carbon support 32 is susceptibleto corrosion due at least partially to the presence of oxygen, water andhigh electrode potential. This corrosion results in carbon oxidationand, as a final stage, collapse of carbon support 32. Corrosion causesmicrostructural derogation and surface chemistry changes, which canresult in an irreversible loss in catalyst performance, and ultimatelyin the complete failure of the fuel cell. Outer carbide thin film 36protects carbon support 32 and reduces or eliminates corrosion. Thus,outer carbide thin film 36 maintains the catalyst performance of theelectrode.

FIG. 3 is a block diagram of method 40 for forming stabilized catalyststructure 30 of FIG. 2. Method 40 includes the steps of depositingcatalyst particles on a carbon support (step 42) and forming outercarbide thin film on the carbon support (step 44).

First, in step 42, catalyst particles are deposited on a catalystsupport structure, such as a carbon support, to form supported catalystparticles. The catalyst particles can be deposited on the carbon supportusing any known deposition method. In one example, the catalystparticles are deposited using a chemical deposition method. In otherexamples, the catalyst particles can be deposited on the carbon supportby a vapor deposition process, an electrochemical deposition process ora thermo-decomposition method. As described above, the carbon supportcan include carbon or modified carbon, and the catalyst nanoparticlescan be selected from platinum, gold, iridium, osmium, palladium, rhodiumand ruthenium and mixtures and alloys thereof Alternatively, the carbonsupport can be acquired with catalyst particles already depositedthereon.

Next, in step 44, a carbide thin film is formed on the carbon support.Microwave irradiation (as will be described with respect to FIGS. 4A and4B) is an example technique that can be used to form the carbide thinfilm. Other known techniques for forming carbide films, such astemperature programmed reaction (TPR), can be used. The desired carbideloading and the formula weight of the carbide compound deposited affectthe thickness of the carbide thin film. Because the formula weights ofcarbide compounds are much greater than that of carbon, a small amountof a carbide thin film greatly affects the weight of catalyst structure30. In fuel cells it may be desirable to have low carbide loading tomaintain a low density catalyst structure. In one example, the carbideloading (the weight of carbide to the weight of carbon support) is notgreater than 50%. In another example, the carbide loading is not greaterthan 30%.

FIG. 4A is a block diagram of method 46 for forming a carbide thin filmby microwave irradiation. Method 46 includes forming a dissolved carbideprecursor solution (step 48), exposing the supported catalyst particlesto a dissolved carbide precursor (step 50), evaporating the solvent(step 52) and microwave irradiating the carbon supports (step 54).

First, a dissolved carbide precursor solution is formed (step 48) bymixing a carbide precursor with a solvent. The carbide precursor can bea salt of any carbide precursor. Example carbide precursors include, butare not limited to, a tungsten salt, a boron salt, a niobium salt, amolybdenum salt, a tantalum salt, a titanium salt and combinationsthereof. Further example carbide precursors include, but are not limitedto, tungsten hexacarbonyl (W(CO)₆), ammonium tungstate (NH₄)₁₀H₂(W₂O₇)₆and sodium borate (Na₂B₄O₇). The carbide precursor can be a monophase oran alloy. The carbide precursor is dissolved into the solvent to form acarbide precursor solution. The solvent can be any solvent whichdissolves the carbide precursor such as water or ethylene glycol.

The amount of carbide precursor dissolved in the solvent depends on thedesired carbide loading. Because the weight of carbides, such astungsten carbide, is much larger than that of carbon, depositing just asmall amount of tungsten carbide on the carbon support significantlyincreases the weight of the carbon support. Additionally, in fuel cells,it is desirable to keep the density of the carbon supports and supportedcatalyst particles low. Therefore, a tradeoff must be made between thecarbide loading and the density of the carbon supports. In one example,the carbide loading (defined as the ratio of the weight of carbide thinfilm to weight of carbon support) is about 50% or less, such as betweenabout 5% and 50%. In another example, the carbide loading is about 30%or less, such as between about 5% and 30%.

Next, the supported catalyst particles are exposed to the dissolvedcarbide precursor in step 50. For example, the supported catalystparticles can be dropped into the dissolved carbide precursor solutionor the carbide precursor solution can be poured over the supportedcatalyst particles. Other methods can also be used to expose thedissolved carbide precursor to the supported catalyst particles.

Next, the solvent is evaporated (step 52). The solvent can be evaporatedby any conventional evaporation technique, such as heating. The solventshould be removed until the supported catalyst particles and carbonsupport are dry. After evaporating the solvent, a film of carbideprecursor is formed on the carbon support. Use of a dissolved carbideprecursor allows a homogenous coating of carbide precursor to form onthe carbon support. In comparison, use of a solid carbide precursor canresult in a non-homogenous or uneven coating.

Finally, the carbon support is subjected to microwave irradiation instep 54. Microwave irradiation increases the temperature of the carbonsupport, which causes the carbide precursor to react with the carbonsupport to form carbide compounds. The irradiation forms a carbide thinfilm on the carbon support. During microwave irradiation, the carbideprecursor will preferentially form carbide compounds on the carbonsupport. The carbide thin film surrounds the catalyst particles andprevents or minimizes the movement of the catalyst particles on thecarbon support.

The resulting carbide thin film is an atomically thin layer. Forexample, the carbide thin film can be between about 1 and 2 atomiclayers thick. In another example, the carbide thin film is less thanabout 1 nm. The carbide thin film can be a homogenous or heterogenouslayer. For example, the carbide thin film can have a uniform ornon-uniform thickness. In another example, the outer carbide thin filmcan be small clusters so that a portion of the catalyst support may notbe covered by the outer carbide thin film.

While the majority of the carbide thin film is formed on the carbonsupport and surrounds the catalyst particles, a small amount of carbideor carbon thin film may also be formed on the catalyst particles.Forming a carbide or carbon thin film on the catalyst particles willreduce the available surface area of the catalyst particles, and thusthe catalyst activity of the catalyst particles. The system can beoptimized to minimize the formation of carbide or carbon thin film onthe catalyst particles by selecting carbide precursors that are morestrongly attracted to the carbon support than to the catalyst particles.

An alternative microwave irradiation method 55 is shown in FIG. 4B.Method 55 is similar to method 46 except for the order of the last twosteps. Method 55 includes forming a dissolved carbide precursor solution(step 48), exposing the supported catalyst particles to a dissolvedcarbide precursor (step 50), microwave irradiating the carbon supports(step 56) and evaporating or filtering the solvent (step 58). Steps 48and 50 are the same as those described above with respect to FIG. 4A.

In step 56, the carbon support is subjected to microwave irradiation,which increases the temperature of the carbon support and causes thecarbide precursor to react with the carbon support to form carbide. Thisforms a carbide thin film on the carbon support. During microwaveirradiation, the carbide precursor will preferentially form a carbidethin film on the carbon support. The carbide thin film surrounds thecatalyst particles and physically anchors the catalyst particles to thecarbon support. The carbide thin film forms a stabilized catalyststructure and prevents or minimizes the movement of the catalystparticles on the carbon support.

After microwave irradiation, the solvent is evaporated or filtered instep 58. The solvent can be evaporated as described above with respectto step 52. Additionally or alternatively, the solvent can be removed byfiltering. In the filtering process, the solvent and the stabilizedcatalyst structures are separated using filter paper. The solvent passesthrough the filter paper, leaving the stabilized catalyst particles onthe filter paper.

FIG. 5 is a cross-sectional view of an alternative stabilized catalyststructure 60, which includes carbon support or core 32, inner carbidethin film 62, catalyst particles 34, and outer carbide thin film 36.Stabilized catalyst structure 60 is similar to stabilized catalyststructure 30 except catalyst structure 60 includes inner carbide thinfilm 62. Carbon support 32 can be any support containing carbon, such asactivated carbon, carbon black, carbon nanowires, mesoporous carbons,surface modified carbons (or modified carbons), carbon nanotubes,graphitized carbon, non-graphitized carbon and combinations thereof.

Inner carbide thin film 62 is deposited on carbon support 32 to form acatalyst support structure. In one example, inner carbide thin film 62contains tungsten carbide, boron carbide, niobium carbide, molybdenumcarbide, tantalum carbide, titanium carbide or combinations thereof.Inner carbide thin film 62 is a homogenous or heterogeneous atomicallythin layer. Inner carbide thin film 62 can have a uniform or non-uniformthickness. In one example, inner carbide thin film 62 has a uniform ornon-uniform thickness between about 1 and 2 atomic layers thick. Inanother example, inner carbide thin film 62 is thicker than outercarbide thin film 36. In a further example, inner carbide thin film 62can be small clusters so that a portion of the catalyst support may notbe covered by carbide thin film 62.

Inner carbide thin film 62 can be formed by any known technique. In oneexample, inner carbide thin film 62 is formed by microwave irradiation,as described above with respect to FIG. 4A and FIG. 4B. In anotherexample, inner carbide thin film 62 is formed by ball milling. Althoughmicrowave irradiation and ball milling are both room temperatureprocesses for forming carbide thin films, other process can be used toform inner carbide thin film 62.

Catalyst particles 34 are deposited on the catalyst support structureformed by inner carbide thin film 62 and carbon support 32. As describedabove, catalyst particles 34 contain a catalyst such as platinum, gold,iridium, osmium, palladium, rhodium and ruthenium and alloys andcombinations thereof Catalyst particles 34 can be a mixture of differentcatalysts.

Catalyst particles 34 can be nanoparticles. In one example, catalystparticles 34 have a diameter between about 0.2 nm and about 100 nm. Inanother example, catalyst particles 34 have a diameter between about 1nm and about 100 nm. In a further example, catalyst particles 34 have adiameter between about 0.2 nm and about 10 nm. In a still furtherexample, catalyst particles 34 have a diameter between about 2 nm andabout 10 nm.

Outer carbide thin film 36 is formed on carbon support 32 after catalystparticles 34 are deposited on the catalyst support structure. Outercarbide thin film 36 anchors catalyst particles 34 to the catalystsupport structure formed by inner carbide thin film 62 and carbonsupport 32. Outer carbide thin film 36 prevents or reduces the movementof catalyst particles 34 on carbon support 32 and inner carbide thinfilm 62. Outer carbide thin film 36 further stabilizes catalystparticles 34. In one example, inner carbide thin film 62 containstungsten carbide, boron carbide, niobium carbide, molybdenum carbide,tantalum carbide, titanium carbide or combinations thereof.

Outer carbide thin film 36 is an atomically thin layer having a uniformor non-uniform thickness. In one example, outer carbide thin film 36 isbetween about 1 and 2 atomic layers thick. In another example, outercarbide thin film 36 is less than about 1 nm thick. In a furtherexample, carbide thin film 36 can be small clusters so that a portion ofthe catalyst support may not be covered by carbide thin film 36.

Outer carbide thin film 36 can be formed by any known technique, such asmicrowave irradiation, as described above with respect to FIGS. 4A and4B. Outer carbide thin film 36 will preferentially form on inner carbidethin film 62, forming around catalyst particles 34. Although notpreferable, a small amount of outer carbide thin film 36 may also formon catalyst particles 34. Depositing outer carbide thin film 36 is notpreferable because outer carbide thin film 36 reduces the surface areaand the catalytic activity of catalyst particles 34. The system can beoptimized to reduce formation of outer carbide thin film 36 on catalystparticles 34 by selecting carbide precursors that are more stronglyattracted to inner carbide thin film 62 than to catalyst particles 34.

The total carbide loading of catalyst structure 60 is defined as theratio of the sum of the weight of carbide thin films 36 and 62 to theweight of carbon support 32. It is desirable to keep the density and thecarbide loading of catalyst structure 60 low when catalyst structure 60is used in fuel cells. In one example, the total carbide loading isabout 50% or less, such as between about 5% and about 50%. In anotherexample, the total carbide loading is about 30% or less, such as betweenabout 5% and about 30%

Outer carbide thin film 36 and inner carbide thin film 62 of catalyststructure 60 stabilize catalyst particles 34. Catalyst particles on thenanoparticle scale, such as platinum nanoparticles, are unstable oncarbon. For example, there is very little or no attraction betweenplatinum nanoparticles and carbon. Thus, platinum nanoparticles migrateand agglomerate on a carbon support forming larger particles. This formslarger catalyst particles and decreases the catalyst activity of thesupported catalyst particles due to the loss of active surface area.

Carbide thin films 36 and 62 prevent or reduce the migration of catalystparticles 34. Compared to inner carbide thin film 62, outer carbide thinfilm 36 greatly improves the stability of catalyst particles 34. Outercarbide thin film 36 anchors or fuses catalyst particles 34 to carbonsupport 32, increasing the stability of catalyst particles 34 andpreventing or reducing agglomerations of catalyst particles 34. Outercarbide thin film 36 is a physical barrier that prevents movement ofcatalyst particles 34.

Further, carbide thin films 36 and 62 of catalyst structure 60 protectcarbon support 32 from corrosion. As described above, carbon supports 32are susceptible to corrosion that results in carbon oxidation and, as afinal stage, collapse of carbon supports 32. Corrosion causesmicrostructural derogation and surface chemistry changes, which canresult in an irreversible loss in catalyst performance. Carbide thinfilms 36 and 62 of catalyst structure 60 protect carbon support 32 fromcorrosion and maintain the catalyst performance of catalyst particles 34and the electrode of which they are a part.

Inner carbide thin film 62 allows the formation of outer carbide thinfilm 34 that otherwise may not be possible. Depending on the method usedto form outer carbide thin film 36, it may be necessary to form innercarbide thin film 62 on carbon support 32 before catalyst particles 34are deposited. For example, platinum catalyst particles can act as acatalyst and prevent the formation of outer carbide thin film 34 when avapor deposition method is used. In such a situation, inner carbide thinfilm 62 can be formed on carbon support 32 before catalyst particles 34are deposited.

Although the present invention has been described with reference topreferred embodiments, workers skilled in the art will recognize thatchanges may be made in form and detail without departing from the spiritand scope of the invention.

1. A catalyst structure for an electrochemical cell, the catalyststructure comprising: a catalyst support structure; catalyst particlesdeposited on the catalyst support structure; and an outer carbide filmformed on the catalyst support structure and surrounding the catalystparticles to anchor the catalyst particles to the catalyst supportstructure, wherein the outer carbide film is preferentially formed onthe catalyst support structure compared to the catalyst particles. 2.The catalyst structure of claim 1, wherein the catalyst supportstructure comprises carbon or modified carbon.
 3. The catalyst structureof claim 1, wherein the catalyst particles are selected from the groupconsisting of platinum, gold, iridium, osmium, palladium, rhodium andruthenium and alloys and combinations thereof.
 4. The catalyst structureof claim 1, wherein the catalyst particles have a diameter between about0.2 nanometers and about 100 nanometers.
 5. (canceled)
 6. The catalyststructure of claim 1, wherein the outer carbide film comprises at leastone of tungsten carbide, boron carbide, niobium carbide, molybdenumcarbide, tantalum carbide, titanium carbide and combinations thereof. 7.The catalyst structure of claim 1, wherein the outer carbide film has auniform thickness of between about 1 to 2 atomic layers.
 8. The catalyststructure of claim 1, wherein the outer carbide film has a non-uniformthickness.
 9. The catalyst structure of claim 1, wherein the outercarbide film is small clusters.
 10. The catalyst structure of claim 1,wherein the catalyst support structure comprises: a core; and an innercarbide film on the core.
 11. The catalyst structure of claim 10,wherein the inner carbide film has a uniform thickness of between about1 to 2 atomic layers.
 12. The catalyst structure of claim 10, whereinthe inner carbide film has a non-uniform thickness.
 13. (canceled) 14.The catalyst structure of claim 10, wherein the inner carbide film isthicker than the outer carbide film.
 15. The catalyst structure of claim10, wherein the inner carbide film comprises at least one of tungstencarbide, boron carbide, niobium carbide, molybdenum carbide, tantalumcarbide, titanium carbide and combinations thereof.
 16. The catalyststructure of claim 1, wherein a carbide loading is about 50% or less.17. (canceled)
 18. A method of forming a catalyst structure, the methodcomprising: providing a catalyst support structure; depositing catalystnanoparticles on the catalyst support structure; and forming an outercarbide film on the catalyst support structure by microwave irradiationafter the step of depositing catalyst nanoparticles, wherein the outercarbide film preferentially forms on the catalyst support structurecompared to the catalyst nanoparticles.
 19. The method of claim 18,wherein the catalyst support structure comprises carbon or modifiedcarbon.
 20. (canceled)
 21. The method of claim 18, wherein the step offorming the outer carbide film comprises: mixing the catalyst supportstructure with a carbide precursor; and microwave irradiating thecatalyst support structure mixed with the carbide precursor.
 22. Themethod of claim 21, wherein the carbide precursor comprises a tungstensalt, a boron salt, a niobium salt, a molybdenum salt, a tantalum saltor a titanium salt.
 23. The method of claim 18, wherein the step ofproviding the catalyst support structure comprises: providing a core;and forming an inner carbide film on the core.
 24. (canceled) 25.(canceled)
 26. The method of claim 18, wherein the step of forming theouter carbide film comprises depositing an amount of carbide on thecatalyst support structure so that the catalyst structure has a carbideloading of about 50% or less.
 27. (canceled)